Electrolytic treatment of nonferrous metals

ABSTRACT

A two-step electrochemical treatment of a nonferrous metal article in which a first coating of electrolytic chromium plate is deposited on a nonferrous metal surface and a second or surface coating containing chromium both as an oxide of chromium and as metallic chromium is electrolytically codeposited over the chromium plate from a chromic acid solution containing an electroplating additive, preferably a halide ion. A duplex coating is formed which imparts very good corrosion resistance with small amounts of coating material and provides a paintable surface without a painting pretreatment.

United States Patent [72] Inventor William A. Carter, Jr.

Gary,1nd. [21] Appl. No. 56,139 [22] Filed July 6,1970 [45] Patented Oct. 26,1971 [73] Assignee Inland Steel Company Chicago, Ill. Continuation of application Ser. No. 685,314, Nov. 24, 1967, now abandoned.

[54] ELECTROLYTIC TREATMENT OF NONFERROUS METALS 5 Claims, No Drawings [52] U.S.Cl 204/41, 204/56 R [51] Int. Cl C23b 5/50, C23b 1 1/00 [50] Field of Search 204/41, 51, 56 R, 58

[56] References Cited UNITED STATES PATENTS 1,813,842 7/1931 Fink et a1 204/41 X Primary Examiner-G. L. Kaplan Attorneyl-libben, Noyes & Bicknell ABSTRACT: A two-step electrochemical treatment of a nonferrous metal article in which a first coating of electrolytic chromium plate is deposited on a nonferrous metal surface and a second or surface coating containing chromium both as an oxide of chromium and as metallic chromium is electrolytically codeposited over the chromium plate from a chromic acid solution containing an electroplating additive, preferably a halide ion. A duplex coating is formed which imparts very good corrosion resistance with small amounts of coating material and provides a paintable surface without a painting pretreatment.

ELECTROLYTIC TREATMENT OF NONFERROUS METALS This application is a continuation of application Ser. No. 685,314, filed Nov. 24, 1967, now abandoned.

present invention relates generally to a nonferrous metal surface which is electrolytically treated to improve the properties thereof, and more particularly to a nonferrous metal article treated electrolytically to improve the surface properties thereof, and to an improved method of electrolytically treating a nonferrous metal article to impart thereto improved surface properties, particularly improved corrosion resistance.

The present invention is an improvement over the invention of electrolytic treatment ofnonferrous metals, such as zinc, tin and aluminum, such as described in the Kitamura U.S. Pat. No. 2,998,361 and in the Yonezaki et al. U.S. Pat. No. 3,257,295. While the products of the foregoing inventions for treating metal surfaces are an improvement in some respects over previous electrolytic and chemically treated metal products, there remains a need for providing a nonferrous metal surface with a high degree of corrosion resistance and paint-adherent properties, which at the same time has good formability and is capable of being produced commercially in a shorter treatment time than is required by the prior art processes.

It is therefore, a object of the present invention to provide an electrolytically treated nonferrous metal surface having improved properties.

It is a further object of the present invention to provide an electrolytically treated nonferrous metal product having improved corrosion resistance and paint adherence without impairing the drawing and forming properties thereof.

It is still another object of the present invention to provide an electrolytically treated sheet steel strip which has a coating of a nonferrous metal to provide good paint-adherent properties and improved corrosion resistance and which can be economically produced on a continuous strip treating line.

It is also an object of the present invention to provide an improved process of electrolytically treating a nonferrous metal surface which imparts thereto improved chemical and physical properties and which can be economically practiced on a continuous treating line.

It is a further object of the present invention to provide a process of treating a sheet steel product which has a coating of a nonferrous metal to provide thereon a two-layered or duplex coating wherein the thickness of either layer can be varied independently of the other so that special properties can be selectively produced.

Other objects of the present invention will be apparent to those skilled in the art from the following detailed description and claims to follow.

It has been discovered that the corrosion resistance and paint-adherent properties of a nonferrous metal surface can be improved by first providing the nonferrous metal surface \"ith a very thin electrolytically deposited metallic chromium plate which is of a thickness normally exhibiting poor corrosion resistance but having good drawing properties and then further treating the surface electrolytically to provide directly over the thin chromium plate a second coating comprised of electrolytically codeposited oxide of chromium-metallic chromium. The duplex electrolytic coating of the present invention is capable of imparting significantly improved corrosion resistance without destroying the formability (i.e. drawing properties thereof) and provides a paint-adherent surface without requiring use of a paint pretreatment. Where desired, heavier coatings of the oxide of chromium-metallic chromium film over a wide range of thickness can be codeposited over the thin chrome-plated panel or strip to provide even greater protection against corrosion.

It has also been found that a product having good adherent properties for siccative coatings, and having in addition very high corrosion resistance, can be produced over a wide range of thickness of the electrolytic codeposited oxide of chromium-metallic chromium coating when the latter is deposited from a chromic anhydride additive containing electrolyte bath directly over the film of metallic chromium. For example, by using a coating solution of the herein described example 1 for codepositing an oxide of chromium-metallic chromium coating and with an electrolytic treatment time of between about 1 seconds at amperes/ftf 100 coulombs per square foot) and 2 seconds at 300 amperes/ft. (600 coulombs per square foot), oxide of chromium-metallic chromium coatings are formed on a chromium-plated galvanized steel sheet or strip which exhibits improved corrosion resistance and paint-adherent surface properties. The thickness of the electrolytically codeposited oxide of chromium-metallic chromium coating is maintained preferably within a range of from about 1 mg./ft. to about 10 mg./ft. total chromium (16.4 mg. chromium per square foot corresponding to a film having a thickness of 1.0 microinches). The corrosion resistance of the thin treated product is very good. An electrochemically deposited coating of oxide of chromium-metalic chromium having a much greater thickness than 10 mg./ft. can, however, be applied over the chrome plate and the corrosion resistance to moisture will be further increased.

The coating of metallic chromium which is applied directly on the surface of a nonferrous metal in the present invention can be applied by any conventional chromium-plating process. However, since the duplex coating of the present invention imparts greatly improved corrosion-resistant properties over a single layer of electroplated metallic chromium, such as produced according to the Uchida et a1. U.S. Pat. No. 3,1 13,845, with or without the Uchida et a1. auxiliary nonelectrolytic treatment using a 2-3 percent solution of sodium dichromate or a 1 percent solution of chromic anhydride, it is possible and preferred in the present invention to use an electroplated metallic chromium coating of substantially reduced thickness in comparison with that which is required in the Uchida et a1. process. The thickness of the metallic chromium plate applied directly on the surface of the nonferrous metal coated steel strip in the present invention generally has a thickness range of from about 0.05 microinches (about 1.0 mg. chromium per square foot) to about 1.0 microinches (about 16 mg. chromium per square foot), and preferably has a thickness of about 0.2-0.5 microinches (about 3 to 8 mg. Cr/ft. Metallic chromium plate within the foregoing range of thickness provides a highly receptive base for the subsequent electrolytically codeposited oxide of chromiummetallic chromium coating and exhibits good formability and workability properties. Also, because the process of the present invention permits the use of an ultrathin film ofmetallic chromium, the time required for chrome plating can be very substantially reduced to a plating time as short as a to 1 second for some applications, and the cost of applying the chromium plate to a nonferrous metal coated steel strip can be substantially reduced and below the cost of applying a heavy coating of nonferrous protective metal, such as zinc or tin. Where processing time and space limitations are not important and for those applications where formability and workability of the treated product are oflittle concern, however, it is possible in the present invention to apply substantially thicker metallic chrome plating above 1.0 microinches in thickness.

In practicing the present invention, a coated metal article, such as a low-carbon steel strip or panel hot dip coated with zinc in the usual manner, is first provided wit a metallic chromium coating applied directly on the galvanized steel surface by immersing the cleaned article as a cathode in a chromium electroplating bath having between about 200 and 500 grams chromic anhydride (CrO per liter, and preferably about 250 grams per liter of the chromic anhydride, and containing a conventional chrome-plating bath additive, preferably about 2.5 grams per liter sulfuric acid (i.e. bath ratio of 100 to 1), while maintaining the bath at a temperature of about 100 F. and passing an electric current through the bath at a current density of between about and 300 am peres per square foot for a period of between about 0.5 and 2.5 seconds, and preferably for about 1 second. The anode can be a conventional lead anode, e.g. an alloy comprised of 93 percent lead and 7 percent tin. Other chromium-plating bath compositions and operating conditions suitable for depositing a like thin metallic chromium film directly on the galvanized steel surface can be used, if desired.

Thereafter, a thin coating of electrolytic oxide of chromium-metallic chromium is codeposited directly on the surface of the metallic chromium plated strip, after thoroughly rinsing the strip with water, by immersing the chromium-plated strip as the cathode in an electrolyte solution comprised of between about 75 to 400 grams chromic anhydride per liter, and preferably about l grams per liter and also having as an essential ingredient an electrolytic plating bath additive of the type used in chromium-plating baths, and which preferably includes halogen acids or halogen salts, such as sodium chloride and sodium fluoride, for providing halogen ions in the bath. The preferred plating additive is sodium chloride used in an amount of between about 0.045 and about 0.36 grams per liter, and preferably using between about 0.15 and 0.2 grams sodium chloride per liter. The ratio of chromic anhydride to halide ion should be at least about 3,7000 to 1 and a maximum ratio of about 4500 to l. The optimum ration is about 750 to l.

When a relatively thin coating of oxide of chromium-metallic chromium is to be codeposited on the surface of the chromium plate, a current density of between about 50 to about 400 amperes/ft, and preferably about 100 amperes per square foot, is applied for a period of about we to 3 seconds, and preferably for about 1 to 2 seconds, while the preferred electrolyte solution is maintained at a temperature of about 140 F. Under the foregoing operating conditions the electrolytically codeposited film of oxide of chromium-metallic chromium generally has a thickness ranging between about 1 mgJft. 2 and mg./ft. total chromium. Heavier coatings can be applied, if desired, by increasing the current density above 400 amperes/ft.

The following equation has been found to give the relationship between the variable operating conditions which determine the thickness of the oxide of chromium-metallic chromium film:

T= plating solution temperature (F.)

P= NaCl in solution (0.01% =0.036 g./l

A current density (amperes/ft?) t= plating times (seconds) It will be evident from the equation that for a given thickness of the oxide of chromium-metallic chromium film the film varies directly with the current density, at time of electrolysis, and the concentration of the bath additive (preferably halide ion), while varying inversely with the temperature of the electrolyte bath. With the aid of the above equation, the operating conditions can be selected which will codeposit on surface film of oxide of chromium-metallic chromium of any desired thickness. And, while the thickness of the surface film of oxide of chromium-metallic chromium codeposited over the thin chrome plate can vary between l to l0 mg./ft. (as total chromium) and provide significantly improved corrosion resistance and good paint adherence, it has been found that a very satisfactory thickness for the surface film consisting of codeposited oxide of chromium-metallic chromium is between about 3-8 mg./ft." (as total chromium) with an optimum thickness of about 7 mg./ft." chromium where it is desired to provide a duplex coating having good paint adherence and good corrosion resistance. The electrolytic overcoating of oxide of chromium-metallic chromium which is electrolytically codeposited on the surface of the chromium plate is formed of about equal amounts of chromium as the oxide and chromium in the metallic form. A film having a thickness of l microinch corresponds to 16.4 mg. chromium per square foot.

Prior to applying the initial metallic chromium coating directly on the surface of a nonferrous metal coating or article, the nonferrous metal surface, if oxidized, can be cleaned by subjecting to light etch in a dilute acid bath.

The following specific examples further illustrate a process of the present invention, but should not be construed to limit the invention to the particular ranges or operating specified.

EXAMPLE 1 A panel of 55 pounds per base box (0.006 inches thick), double-reduced, bright black plate with its surface having an electrogalvanized coating weighing 0.025 oz./ft. per side, is given a light etch in a dilute sulfuric acid bath to remove any oxide film on the surface thereof. The panel is then thoroughly rinsed with water and while still wet is passed into a metallic chromium-plating bath containing 2500 grams per liter chromic anhydride and 2.5 grams per liter sulfuric acid. With the galvanized metal panel as the cathode a current of about 200 amperes per square foot is passed through the bath for a period of 2 seconds and with the bath at a temperature of l l0 F. to effect deposition of a layer of metallic chromium having a film thickness of about 0.45 microinches. The panel is thoroughly rinsed with water and while wet is passed directly into a second electrolytic bath containing 100 grams per liter chromic anhydride Cro3) and 0.15 grams per liter sodium chloride. With the chromium-plated panel as the cathode, an electric current having a current density of 100 amperes per square foot is passed through the electrolyte solution for a period of 1 second while the bath is maintained at a temperature of about 140 F. to effect cathodic codeposition of a film of oxide of chromium-metallic chromium. The panel on being withdrawn from the second electrolyte solution is thoroughly rinsed with water and dried. If desired, a protective oil or other protective coating can be applied to the dry panel.

EXAMPLE 2 Panels of black plate having electroplated on the surface thereof a 0.0l-mil zinc coating were chromium plated by immersing the panels in a standard chromium-plating bath at a temperature of 120 F. for a period of5 seconds with a current of amperes per square foot being applied therethrough. The chromium-plated surfaces were rinsed with water and immersed as the cathode in a chromic anhydride bath having the composition specified in example 1 and a temperature of 120 F. while applying current of l amperes per square foot for a period of 2 seconds. After rinsing and drying the panel withstood exposure in a standard salt fog test for 240 hours before failure; whereas the control zinc-coated panel without the above duplex coating failed in the salt fog test after 120 hours.

EXAMPLE 3 A continuous strip of black plate having a galvanized coating 0.2 mils thick was treated on a continuous strip processing line by first passing the strip continuously through a conventional alkaline cleaning solution. After thoroughly rinsing the strip with water at a temperature of about 110 F., the strip was continuously passed through a metallic chromium-plating bath having a temperature of 110 F. containing 250 grams per liter of chromic anhydride and 2.5 grams per liter of sulfuric acid. While the strip was made the cathode in the plating bath a current of 200 amperes per square foot was passed therethrough for a period of 2 seconds to effect deposition of a layer of metallic chromium having a film thickness of about 0.45 microinches. After thoroughly rinsing with water the strip was passed directly into a second electrolyte bath at a temperature of about ll0 F. containing grams per liter chromic anhydride and 0.18 grams per liter sodium chloride. and with the strip as the cathode an electric current was passed therethrough having a current density of 200 amperes per square foot for a period of 2 92 seconds to effect codeposition ofa film of oxide of chromium-metallic chromium having a thickness of about 7 milligrams per square foot chromium. The strip on being removed from the bath was thoroughly rinsed with water and dried.

The strip treated in the foregoing manner exhibited good corrosion resistance when subjected to conventional salt fog test wherein the treated strip resisted exposure for 880 hours.

while the control strip failed after 500 hours exposure. The paint-adherence properties of the treated strip were good without employing the customary phosphate paint pretreatment. Chemical analysis of the surface coating provided in the foregoing process indicated that chromium content of the coating was about 35 percent as trivalent chromium and about 65 percent as metallic chromium.

While the present invention is in no way dependent on any theory explaining the mechanism by which the improved results are produced, it appears from extensive X-ray diffraction studies and microscopic examinations of the duplex coatings of the present invention that under the herein-disclosed operating conditions two distinct films are formed, one consisting of a thin but relatively uniform porous metallic chrome plate formed directly on the surface of the nonferrous metal article or coating in the chromium-plating bath, and a second or outer film electrolytically deposited from the second chromic anhydride containing bath which contains both oxide of chromium and metallic chromium in about equal proportions with the other film being nonuniformly distributed over the surface of the chrome plate and having a large proportion thereof concentrated as islands in a random manner. It is believed that the random concentrations or islands are formed at the pore or imperfections in the chrome plate and provides a more uniform, impervious surface which resists corrosion. It is also possible that having metallic chromium in the surface film may further contribute to the improvement in the paint adherence of the resultant duplex coating. The fact that the outer coating of oxide of chromiummetallic chromium must not exceed a maximum thickness in order to avoid interfering with the inherent good paint adherence of the chromium plate, however, suggests that the underlying chrome plate exerts a definite paint-adherent effect through the outer coating when the thickness of the outer coating is restricted to the herein-specified limits.

Others may practice the invention in any of the numerous ways which are suggested to one skilled in the art by this disclosure, and all such practice of invention are considered to be a part hereof which fall within the scope of the appended claims.

I claim:

1. A process of electrolytically treating a surface of a nonferrous metal base to improve the corrosion resistance and lacquer adherence of said base which comprises; cathodically electrolizing said nonferrous metal surface in an aqueous bath containing hexavalent chromium to deposit on said surface a thin metallic chromium plate having a weight between about 1 and 16 milligrams chromium per square foot, and thereafter making said chromium plate cathodic in an aqueous electrolyte bath consisting essentially of between about 75 and grams per liter chromic anhydride and sodium chloride with said chromic anhydride and the chloride ion of said sodium chloride being present on a weight basis in a ratio of between about 3,7000 to l and 450 to 1 while passing a current through said bath at a current density of from 50 to 400 amperes per square foot for a period of from about it to 3 seconds at a bath temperature between about 1 10 and F. to effect electrolytically codepositing metallic chromium and oxide of chromium as a homogeneous film on the surface of said chromium plate.

2. A process as in claim 1, wherein said electrolyte bath contains 100 grams. chromic anhydride per liter and said sodium chloride is present in said electrolyte bath in an amount between about 0.15 and 0.2 grams. per liter.

3. A process as in claim 1, wherein said ratio ofchromic anhydride to chloride ion on a weight basis is about 750 to l.

4. A process as in claim 1, wherein a current density of about 100 amperes per square foot is applied for a period of about 2 seconds.

5. A process as in claim 1, wherein said film of codeposited metallic chromium and oxide of chromium is maintained at a weight of between about 3 and 8 milligrams chromium per square foot.

i t l mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,616,303 Dated February 3, 1972 Inv n fl William A. carter, Jr.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 3, line 20, "3,7000" should read --3700--; line 21, "4500 should read --450-; Col. 4, line 3, "operating specified. should read --operating conditions specified.--; line 9, "0.025" should read --0.25-; line 13, 2500" should read -250--; line 38, "of amperes" should read --of 180 amperes-; line 68, 292 seconds" should read --2 seconds; Col. 6, Claim 1, line 14, "100" should read --400--; line 17, "3,7000" should read -3700--.

Signed and sealed this 9th day of May 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTISGHALK Attesting Officer Commissioner of Patents 

2. A process as in claim 1, wherein said electrolyte bath contains 100 grams chromic anhydride per liter and said sodium chloride is present in said electrolyte bath in an amount between about 0.15 and 0.2 grams per liter.
 3. A process as in claim 1, wherein said ratio of chromic anhydride to chloride ion on a weight basis is about 750 to
 1. 4. A process as in claim 1, wherein a current density of about 100 amperes per square foot is applied for a period of about 2 seconds.
 5. A process as in claim 1, wherein said film of codeposited metallic chromium and oxide of chromium is maintained at a weight of between about 3 and 8 milligrams chromium per square foot. 